Friday, August 23, 2013

No, not this Monitor, this Monitor. Ever since it’s introduction, it’s been the subject of both scorn and praise… ok, ok, scorn. Recently, during our current field test for the next version of RAD Studio*, one of our long-time field-testers posted a reference to this excellent blog post by Eric Grange relating to the performance of the TMonitor vs. the Windows Critical Section. In that post, Eric makes a strong case against TMonitor. Since I’m somewhat personally invested in the TMonitor (read: I wrote it), I took that post as a challenge. I would have responded directly were it not for the fact that he’s closed comments for that post. No biggie. For the record, while Eric and I don’t always agree, he always has well reasoned and thorough arguments.

Eric published a simple test case to demonstrate what he was talking about. I took that test case and began to dig into it. First of all I looked into what Windows was doing in the critical section code down in the kernel. A quick spelunk with the CPU view was rather revealing. In this specific case, the critical section wasn’t ever actually blocking on a kernel-level object. In other words it was doing a busy-wait. Under the high-contention test case that Eric presented, the busy-wait was the magic that served to blow TMonitor out of the water! As a baseline here’s my result with Eric’s app unchanged: